1. Field of the Invention
The present invention relates to an image processing apparatus and a control method therefor and, more particularly, to an image processing apparatus for forming a color image by electrophotography, and a control method therefor.
2. Description of the Related Art
Generally, when images are repeatedly formed in a color image forming apparatus, the density of output images gradually decreases. To prevent a drop in density of output images below the guaranteed quality level, the color image forming apparatus is set to forcibly interrupt printing and to execute density control of output images when the print number reaches a predetermined number.
FIG. 11 shows conventional density control. In FIG. 11, reference symbols J1, J2, and J3 denote print jobs. One job is made up of at least one continuous printing sequence. When the total print number by jobs reaches N2, density control a is executed to set the status representing "density control in progress" to level H. To obtain information for halftone density control, the image density must be measured. When a command instructing execution of density measurement is received during execution of density control, density measurement b is done upon completion of density control. The status indicating that density control is underway is reset upon completion of the density measurement.
The above-described density control technique in the conventional color image forming apparatus suffers the following problems. More specifically, when the print number reaches a predetermined number (N2), printing is forcibly interrupted even during continuous printing, like job J3 shown in FIG. 11, in order to execute density control. Several images (n images) scheduled to be continuously printed are left unprinted. Therefore, the throughput of the interrupted job (J3) greatly decreases owing to the n unprinted images.
Although the density of output images gradually decreases, the change is slow. In a given group of continuously printed output images, any change in density is not visually noticeable. However, when continuous printing is interrupted, and density control of output images is executed, the density difference between images output before and after the interruption may visually stand out.